1. Field of the Invention
The present invention relates to arbitrary function generators and the implementation of such function generators in a manner that allows ease of programming and a high degree of interaction with the target application. Specifically, the arbitrary function generator of the present invention provides radio frequency (RF) control and gradient control in a Magnetic an Resonance Imaging (MRI) system.
2. Description of Related Art
In MRI, the subject is placed in a strong static magnetic field and exposed to RF waves. The subject absorbs the RF energy and then re-radiates the RF over a short period. The re-radiated RF signal can be detected and A used to construct an image of the subject. MRI is in general clinical use, but is still an active research area.
To effect imaging, it is necessary to change the magnetic field during the imaging sequence. Special gradient coils are used to produce changes in the magnetic field in the X, Y, and Z dimensions. Gradient amplifiers supply the individual electric currents through the gradient coils. An amplifier""s current output is proportional to an external control value that can be changed rapidly. The external control value may be a small voltage or current, or may be supplied directly in digital form. The device used to produce the time varying control value is a function generator.
The generated RF signal must also be precisely controlled in frequency, phase, amplitude, and duration. Typically, a separate frequency synthesizer is used to set the base frequency. Then one or two function generators are used to modulate the frequency and other parameters.
Because the gradient and RF functions control the timing of an MRI sequence, it is often convenient for the functions to be tagged with markers that are then used as triggers for other parts of the system. For example, a marker can be placed in the read gradient function to tell the RF receiver when to begin sampling the input RF signal.
There are a number of commonly used basic sequences of functions for producing images, for example, Spin Echo and Gradient Echo. From these sequences come the logical names often given to the orthogonal gradient dimensions: Read, Phase, and Slice. These logical gradients may be arbitrarily rotated before being applied to the actual gradient devices which are referenced by X, Y, and Z.
For a given subject, each sequence must still be tuned and adjusted in order to get a satisfactory image. More power might be required to necessitating an increase in the RF function amplitude. A particular part of the function may need to be delayed in order to change contrast. The imaging point may have to be moved to home in on a particular anatomical feature. These changes may require the interaction with an operator viewing previous images. The function generator reprogramming time adds to the delay from operator change to image display so that minimizing this time is desirable.
Many function generators rely on hardware implementations so that functions are not easily modified or reprogrammed. Hardware xe2x80x9cpre-codingxe2x80x9d is commonplace such that a dedicated microprocessor with static RAM may hold the function descriptions that are reused. However, where it was once reasonable for function generators to conserve resources by reusing significant portions of a sequence, some newer sequences are unique end-to-end and have extreme demands for function memory that need to be accommodated. In order to meet minimum requirements, function generators often employ specialized signal processors embedded into the architecture. when the demand for improved performance arises, costly and time-consuming reengineering delays are encountered in engineering the new technology in the hardware.
Magnetic fields in gradients experience memory of their recent history, or hysteresis. It is commonplace in MRI imaging to compensate for this by modifying the control function pattern driving the amplifiers. This is often done in special hardware connected between the function generators and the amplifiers. Once again, the reliance on hardware to perform the functions makes modifications and reprogramming more difficult.
There is a need for a function generator that reduces reliance on hardware dependencies and may be reprogrammed quickly and efficiently. In addition, it is important to provide a user interface that simplifies the description of functions so that the function generator can respond appropriately.
The present invention, an arbitrary function generator, is an instrument for producing one or more waveforms for arbitrary applications wherein a general-purpose waveform production capability is customized to meet the demands of specific applications. Using the present invention, waveforms may be scaled arbitrarily in amplitude and in time. The arbitrary function generator of the present invention is implemented as a standalone system that may be integrated into a MRI control system. In a preferred embodiment of the present invention, a software system comprising a delivery component, a configuration component, a scaling component, and a triggering component is used to implement the features and functions of the present invention. The various software components communicate in accordance with a shared memory database. Data structures comprising waveform descriptions and tuning parameters are made available to the various software components through the shared memory database.
An operator working at a console may enter waveform descriptions and tuning parameters initially in textual form. The textual waveform descriptions are then translated into binary waveform descriptions. The binary waveform descriptions may be further scaled, rotated, etc. virtually in real-time. The functions related to the waveform descriptions may then be delivered to a hardware synchronization board that controls the device associated with the function generator.
When used in conjunction with a MRI control system, the present invention provides an operator with greater flexibility in configuring new applications and imaging sequences. Using the present invention, an operator is not required to reprogram gradient or RF functions as with prior art systems. The present invention provides an operator with greater control and also dynamic control over the functions as they may be scaled in amplitude and in time and rotated in accordance with operator instructions.